1. Field of the Invention
The subject invention generally pertains to refrigerant systems and more specifically to unit-mounted starters for centrifugal chillers.
2. Description of Related Art
Typical centrifugal chillers are refrigerant systems that includes a centrifugal compressor driven by an electric motor. The cooling capacity of the chiller may range from a few tons to a thousand tons or more. The smaller units often operate at 480 volts or less and can be turned on and off by a full-voltage starter, which selectively applies or removes the full supply voltage to the motor.
Larger chillers that can provide at least 300 tons of cooling are often powered by higher voltages of 2,000 volts or more. The higher voltage means less current is needed for a given size motor, and less current means lighter gauge power supply lines and/or less power losses in the wires. However, suddenly applying or removing a full 2,000+ volts across the motor leads can place a significant load on a facility""s electrical supply system and can be quite a shock to the motor, compressor, and the contactors that switch the power on and off. Consequently, such units often employ some type of reduced-voltage starter, whereby reduced voltage is applied at startup, and full voltage is applied shortly thereafter.
Perhaps the three most common types of reducedvoltage starters are the primary reactor, autotransformer, and solid-state starter. A primary reactor includes a resistor in series with each power line leading to the motor. A set of contacts serves as a shunt across each resistor to effectively add or remove the resistor from its respective line. When the contacts open, the resistors reduce the line voltage to the motor for reduced-voltage starting. When the contacts close, the current bypasses the resistors to apply full voltage to the motor during normal operation. An autotransformer includes a transformer with multiple leads that a set of contacts selectively taps to apply full-voltage or reduced-voltage across the motor leads. A solid-state starter includes at least one solid-state electrical xe2x80x9cswitchingxe2x80x9d device (e.g., SCR, triac, diac, power transistor, etc.) that interrupts or changes the waveform of the power leading to the motor to deliver less power to the motor at startup, and full power afterwards.
For typical higher voltage chillers, a full-voltage starter and/or a reduced-voltage starter are often contained within a single enclosure that is installed off the unit in an electrical room or in another area within the building served by the chiller. The starters are often installed alongside other electrical equipment of the building. Main electrical supply lines of the building lead to the full-voltage starter, which in turn feeds the reduced-voltage starter. Wires from the reduced-voltage starter convey the electrical power to the motor. The reduced-voltage starter provides reduced voltage to the motor at startup and full voltage afterwards.
Installing a higher voltage chiller separate from its full-voltage and reduced-voltage starters adds cost and time to the initial installation of the chiller because of the extra electrical work that is required in the field to install the two starters. Unit-mounted starters are commonly found on relatively small chillers; however, larger chillers are different.
Larger chillers generally have larger starters, yet they have less room for them because of shipping rules that limit a chiller""s overall physical size. Although some full-voltage starters have been mounted to larger chillers (i.e., at least 300 tons and over 2,000 volts), such large chillers have not been known to include both a unit-mounted full-voltage starter and a unit-mounted reduced-voltage starter. The physical size of such chillers, even without any unit-mounted starters, is often already pushing the allowable envelope for normal shipping. And starters for larger chillers are quite heavy and bulky, which can make them difficult to mount to a chiller.
Moreover, certain areas on a chiller should be left clear of obstructions to provide room for maintenance, lifting, and to meet applicable codes and regulations. Other areas of the chiller may be taken up by chilled water piping, condenser water piping, a purge unit, a control panel, an economizer, and various sensors. In the end, there is often insufficient room left on the chiller for installing bulky unit-mounted starters.
Consequently, there is a need for providing a strategic location and method for mounting both a full-voltage starter and a reduced-voltage starter to a relatively large centrifugal chiller while still being able to use standard modes of shipping.
To provide a large centrifugal chiller with both a full-voltage starter and a reduced-voltage starter, it is an object of some embodiments of the invention to install the two starters in their own separate enclosure rather than in the same one. For larger chillers, it can be easier to find room to mount two smaller enclosures than to find room for a single, much larger enclosure.
Another object of some embodiments is to reduce the time and cost of installing a large centrifugal chiller.
Another object of some embodiments is to mount both a full-voltage starter and a reduced-voltage starter to a large centrifugal chiller without exceeding maximum dimensions of standard shipping methods.
Another object of some embodiments is to create a convenient user interface with a large centrifugal chiller by mounting a control panel and a full-voltage motor starter on one side of the chiller""s motor and mounting a reduced-voltage starter on the opposite side of the motor. This arrangement can make the control panel and full-voltage starter more accessible than the reduced-voltage starter, which can be a benefit in cases where the reduced-voltage starter requires less attention by operators and service personnel.
Another object of some embodiments is to mount a relatively heavy starter enclosure atop a condenser or evaporator shell and to mount a lighter starter enclosure in a more off centered position to one of the shells, whereby the heavier enclosure receives substantial vertical support, and the lighter enclosure is readily accessible.
Another object of some embodiments is to place an evaporator shell and condenser shell in a staggered arrangement that provides room for two separately mounted motor starters.
Another object of some embodiments is to route a chiller""s electrical power first into a full-voltage starter enclosure, then into a reduced-voltage starter enclosure, and then back to the full-voltage starter enclosure before delivering the power to the motor. This allows the full-voltage starter enclosure to be placed in a location that is accessible to a user, adjacent to the motor leads, and convenient for feeding the main power supply lines to the chiller. Also, the reduced-voltage starter enclosure can be placed in a more out-of-the-way location that does not interfere with operation or servicing of the chiller.
One or more of these and/or other objects of the invention are provided by a rather large centrifugal chiller that includes both a unit-mounted full-voltage starter and a unit-mounted reduced-voltage starter. The chiller has a rated cooling capacity of at least 300 tons and is driven by a motor that is powered by at least 2,000 volts. The chiller includes an evaporator shell and a condenser shell in a staggered arrangement to provide a combined height of between 6 and 12 feet and a combined width of between 5 and 8.5 feet.